> as they seem to never restock most of their products

There is a product development strategy (I'm not sure if there's a formal name for it) where you're given a lead on a finite-but-large supply of parts you can acquire for absurdly cheap; so you buy the batch; develop and price a product around the part; market your product until you run out of the part; and then, rather than switching over to paying retail for the parts and pricing up your product, you just put your product on indefinite restock hiatus (only ever to be fulfilled if you happen to get another lead on a cheap supply of that same part.)

Usually, though, you get a lead on a cheap supply of a different part; and so the cycle begins again.